What is acetylcholine classified as?

It is a neurotransmitter

Acetylcholine is a neurotransmitter that is responsible for making muscles move. For example, when you move your arm, a flow of acetylcholine is released, and when you stop moving your arm, this flow is cancelled by the chemical acetycholine esterase.

This is important because if the flow of acetylcholine is not stopped, then you cannot move your arm back to its original position.